Dawn detected expanding patches of ice on the walls or floors of several craters attributed to a seasonal ice cycle. Astronomers watched one such cycle in Juling Crater, located in the southern hemisphere. According to Dawn’s chief engineer and project manager, Marc Rayman, “In southern hemisphere summer there is greater heating on the floor of that crater, so that warms the ground and releases water vapor. The vapor comes up and condenses on the cold north wall.” Researchers charted one area of ice that grew by hundreds of acres: “It’s water molecules being transported from one location to another,” Rayman says.
Ice has been observed all across Ceres. But Ceres is too close to the Sun for ice to remain stable on the surface. So, when ice is observed, it’s a strong indication of some kind of activity. “Ceres is clearly a geologically active world,” Rayman says.
A leaky world?
Geysers are one mode of transporting salts or condensing water from Ceres’ interior to its surface. The brilliant deposits may represent sites of ancient cryovolcanism, where water vapor leaked or exploded through the crust, forcing out material from subsurface aquifers or seas.
Some activity may continue even now. In 2014, the European Space Agency’s (ESA) Herschel Space Observatory detected clouds of vapor escaping from two distinct spots on Ceres at a rate of 13 pounds (6 kilograms) per second. This observation was the first confirmation of water plumes in the asteroid belt. Scientists theorized the vapor came from ice sublimating on the dwarf planet’s surface.
But the Herschel observations are quite difficult to interpret. Revised assessments have called the results into question. For its part, Dawn did not see enough surface ice to account for what Herschel detected. But if there is subsurface ice, some of it could be a source of water that makes its way up through the ground.
Another possibility is that increased solar activity produced the transient water vapor that Hershel detected. “Say the Sun produces a coronal mass ejection, so a large number of energetic solar particles impinge on the surface and themselves liberate water molecules,” Rayman says. Dawn carried an instrument to detect such high-energy particles from the Sun to investigate this possibility. Astronomers set up a coordinated international campaign between Earth-based telescopes and Dawn, but solar activity was simply too low to demonstrate whether the Sun’s activity freed water from the surface.